In power electronics, power quality, power density, and efficiency are among the most significant considerations when optimizing the conversion of power from one form to another. For example, power quality is a significant factor when interfacing with the electric grid and electric machines. Maintaining high power quality can be important to avoid issues, such as electromagnetic interference (EMI) pollution, flicker, and shortened life of electric machines due to high current harmonics and dv/dt stresses. Power converters play an important role in this process.
There are generally two methods to achieve high power quality and density in power electronics: increasing switching frequency and multi-level topology. Increasing the switching frequency has limitation, especially for high power and/or medium voltage converters, due to higher losses of power semiconductors associated with higher switching frequency and intrinsic limit of switching speed for high voltage and high power semiconductor devices. Thus, for high voltage and high power applications, multi-level topology is a more effective approach than increasing switching frequency.
Multi-level converter topologies more easily achieve high power quality, high density at higher efficiency. Interfacing with AC electric source and/or load, such as utility grid and electric machines, multi-level converters emulate alternating current (AC) output waveforms by providing multiple voltage levels at the output of the converter. Consequently, switching frequency can be reduced due to lowered output harmonics as result of the multi-level output. Several conventional multi-level topologies and control solutions are widely used in the industry.
One conventional multi-level topology is a three-level neutral point clamped (NPC) topology, which has been the industry's workhorse for over a decade, especially for output voltage below 3.3 kV. However, expanding NPC technology beyond three-levels, in order to achieve higher power quality or for higher voltage applications, represents a significantly increased complexity, thus impractical for wide industry use.
In order to achieve higher than three multi-level output, one has to find ways to couple multiple converters. There are fundamentally two ways for coupling multiple converters—(a) coupling through magnetic components, or (b) coupling through (flying) capacitors.
There are two approaches (i.e., topologies) for coupling multiple converters through magnetic components to realize multi-level converters. A first approach includes multiple converters, generally connected in parallel (or shunt), and coupled with interphase reactors or transformers. This first approach is controlled with interleaved pulse width modulation (PWM) and produces multiple output voltage levels. Drawbacks of this approach include circulating current among the parallel coupled converters, ultimately leading to higher losses, lower semiconductor utilization, and increased control complexity.
A second approach includes multiple single-phase H-bridges (either two-level or three-level H-bridges) connected in series (or cascaded), where each of those single-phase H-bridges are connected to isolated DC links. Due to galvanic isolation provided by a multi-winding transformer, the H-bridges can be coupled together directly with cascaded connection to produce multi-level output voltages correspondingly. Multi-winding transformers, however, are complex and bulky. Also, this approach is difficult and costly to be tailored for four-quadrant operation.
Generally, to process same amount of power, capacitors and power semiconductors tend to have higher density and lower cost than that of magnetic components. Therefore, in comparison to coupling multiple converters with magnetic components, coupling multiple converters with (flying) capacitors provides better power density and efficiency at a lower cost.
Modular multi-level converters (MMC) are yet an additional and widely used capacitor based topology. A number of modular H-bridges are cascaded directly to provide multiple output voltage levels, each one having its own floating DC link capacitors. The voltage levels of these DC links are tightly regulated, using the load current among multi-phases of the cascaded bridge legs. The size of the DC-link capacitors is inversely proportional to the fundamental frequency of the corresponding AC terminal. This solution, therefore, is not optimal for low and variable frequency applications, such as motor drives, due to fairly large floating DC link capacitors.
A better approach than the conventional approaches described above includes coupling multiple converters together through flying capacitors to provide multiple output voltage levels. Voltages across the flying capacitors are regulated every switching cycle. As such, the capacitor size is inversely proportional to switching frequency, instead of the fundamental frequency of the AC source or load. Since the switching frequency is typically more than 30-50 times higher than fundamental frequency, floating or flying capacitor size can be effectively reduced. A further increase of power density, and a reduction in cost, can be thus achieved.